Expert rifts are level 52 rifts designed for a group of five or more ascended. They have near the same difficulty as a tier one expert level dungeon. As of 1.3 there are six total expert rifts, one for each rift element. Abbess Katia for the Guardians and Rift Mob: for the Defiants each offer a daily quest to complete one of the six expert rifts.
Each expert rift will have at least one, but likely more, timed phases. There are no penalties for bringing more than five ascended and doing so can make the encounter much easier. The standard group configuration of one tank, one healer, and three DPS usually work well for these encounters. If the group has experience with tier one or tier two expert dungeons then they should be able to handle the expert rifts without too much trouble.
During the encounters the group has a chance to collect from two separate treasure chests. The first will spawn at the end of phase three of the rift and contain a random tier one level blue essence. The second chest will spawn after the final stage of the rift has been completed and it will contain a random tier one blue essence and a random piece of tier one quality gear. This second chest will also contain four Crystal Soucestones and the item that is needed for the daily quest, for any ascended who have the quest in their logs.